The more powerful the motor, the smaller the pinion you need to help control the heat and keep the motor operating in the proper rev range.
That being said, Castle's stuff tends to run cool and that motor should be fine geared like an 8.5 which is typically ran at @21 pinion anyway.
If you do find yourself needing more pinion gears in the future, I've always been pleased with the
Robinson brand of gears.